DE 10 2008 053 069 A1 describes a free piston apparatus having two piston devices and pistons positioned in opposed piston arrangement. The piston of the first piston device unblocks at the bottom dead center an inlet opening and an outlet opening for a cross scavenging of the combustion chamber. The second piston device may be operated such that the second piston, depending on load, optionally unblocks or blocks an outlet opening spaced axially apart from the opening stated above when the piston of the second piston device adopts the bottom dead center. In addition to the cross scavenging, a uniflow scavenging of the combustion chamber may also be carried out, depending on load.
A free piston apparatus in which a cross, uniflow, and loop scavenging of the combustion chamber may be carried out is described in DE 10 2008 053 068 A1.